Uninterruptible power supply to atmega

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Hello,

i'm trying to design a circuit so that in the event of a power outage, the atmega will be powered by a battery back up. What is the maximum amount of time that an atmega64 can be ON (without turning off / resetting) if its power supply is interrupted?

I was thinking of the 2nd circuit attached, but i think that in this configuration the circuit will be always drawing some current from the battery even if there is external power. So i'm thinking of another circuit with a switch (maybe a single pole, double throw switch) -- sthing similar to the 1st circuit attached but with DC power and w/o the inverter --- so that if there's a power outage, then the switch would connect to the battery to power the chip... is this even possible? would the switch be fast enough to avoid the atmega to reset?

any other thoughts / advice about how others have solved a similar problem are welcomed and appreciated!

thank you!

EDIT. sorry, i should have posted this general post on the avr forum...
- Eric

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Most transistors will only survive a few volts base-emitter reverse bias (typically 5V). In you circuit, when +15V is off, there will be about 8V reverse base-emitter bias. That could permanently damage the transistor. Why not just another diode?

Jim

 

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oh yes, a diode replacing the transistor would work :P but i think there would still be the problem that the battery would always be draining even if there's external power...

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i'm not sure about

Quote:
i think there would still be the problem that the battery would always be draining even if there's external power...

would this be the case?

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No, it would, if anything, be "charged", not drained. When the external supply is present, the only battery current is reverse leakage in the diode. And, that current is in the direction to charge the battery, not discharge it. However, for most diodes, the leakage current is so small that its effect in a case like this is zero.

By the way, this posting has nothing to do with gcc.

Jim

 

Until Black Lives Matter, we do not have "All Lives Matter"!

 

 

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This subject has been up several times before, so a search might turn up earlier discussions, schematics, component selection etc. Not sure about good search terms but "battery backup" might be one of'em. Maybe combine with "diode"...

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Quote:
By the way, this posting has nothing to do with gcc.

yes, i mentioned before

Quote:
EDIT. sorry, i should have posted this general post on the avr forum...

sorry about that. and thank you for your answers! : )

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one more question Jim, if the external supply would be 9 V, then yes, both the supply and the battery would power the circuit, correct?

with a +15V supply, the reason why the battery is not being discharged is because the supply is at a higher voltage?

thank you!
- Eric

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I'm not sure I can spot what on Earth this thread has to do with GCC (subject of this forum) so I'll move this to General Electronics.

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As of January 15, 2018, Site fix-up work has begun! Now do your part and report any bugs or deficiencies here

No guarantees, but if we don't report problems they won't get much of  a chance to be fixed! Details/discussions at link given just above.

 

"Some questions have no answers."[C Baird] "There comes a point where the spoon-feeding has to stop and the independent thinking has to start." [C Lawson] "There are always ways to disagree, without being disagreeable."[E Weddington] "Words represent concepts. Use the wrong words, communicate the wrong concept." [J Morin] "Persistence only goes so far if you set yourself up for failure." [Kartman]

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thanks Johan!

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Don't thank me - thank the search function here on 'freaks. You could have been in the know 20 hours ago...

As of January 15, 2018, Site fix-up work has begun! Now do your part and report any bugs or deficiencies here

No guarantees, but if we don't report problems they won't get much of  a chance to be fixed! Details/discussions at link given just above.

 

"Some questions have no answers."[C Baird] "There comes a point where the spoon-feeding has to stop and the independent thinking has to start." [C Lawson] "There are always ways to disagree, without being disagreeable."[E Weddington] "Words represent concepts. Use the wrong words, communicate the wrong concept." [J Morin] "Persistence only goes so far if you set yourself up for failure." [Kartman]